Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of exhaust aftertreatment systems on engines to reduce harmful exhaust emissions is increasing. For reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems use ammonia to reduce NOx in the exhaust stream. When just the proper amount of ammonia is available at the SCR catalyst under proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. Ammonia is an extreme irritant and an undesirable emission. Accordingly, slips of even a few tens of ppm are problematic. Additionally, due to the undesirability of handling pure ammonia, many systems utilize an alternate compound such as urea, that vaporizes and decomposes to ammonia in the exhaust stream. Presently available SCR systems treat injected urea as injected ammonia, and do not account for the vaporization and hydrolysis of urea to component compounds such as ammonia and isocyanic acid. As a result, the urea can decompose to ammonia downstream of the SCR catalyst, which can cause ammonia slip and may result in less ammonia being available for NOx reduction than the control mechanism estimates, resulting in higher NOx emissions at the tailpipe.
Ammonia slippage also poses a problem for NOx sensor technology, which has been found to exhibit ammonia cross-sensitivity. Thus ammonia slippage could be erroneously interpreted as NOx emission, thereby calling for increased reductant dosing via a feedback control loop. The resulting positive feedback in that case would have the undesirable effect of further increasing rather than decreasing the ammonia slippage.
SCR systems that utilize urea dosing to generate ammonia depend upon the real-time delivery of urea to the SCR catalyst as engine NOx emissions emerge. Urea dosers have relatively slow physical dynamics compared to other chemical injectors such as hydrocarbon injectors. Therefore, urea doser dynamics can substantially affect an SCR controls system.
Some currently available SCR systems account for the dynamics of the urea dosing and the generally fast transient nature of the internal combustion engine by utilizing the inherent ammonia storage capacity of many SCR catalyst formulations. One currently available method introduces a time delay at the beginning of an engine NOx spike before urea dosing begins (or ramps up), and a time delay after the NOx spike before urea dosing ends (or ramps down). Ordinarily, an engine NOx spike will cause a temperature increase in the exhaust gas and SCR catalyst, which may result in the release of stored ammonia on the catalyst. This is especially true when engine power output is used as a substitute for directly estimating engine NOx emissions. The ammonia release provides ammonia for reducing engine out NOx while delaying urea injection prevents excess ammonia from slipping out the exhaust. On the NOx decrease, normally the temperature of the engine exhaust and SCR catalyst decrease, and therefore continued urea injection (the delay before ramping down urea injection) provides ammonia to store on the SCR catalyst and recharge the catalyst.
In many ordinary circumstances, the time delay method causes desirable results in the SCR catalyst. However, in some cases the time delay method can produce undesirable results and even responses that are opposite from an optimal response. For example, a decrease in EGR fraction for any reason causes an engine out NOx spike with a decrease in exhaust temperature. In a time delay system utilizing engine-out power as a substitute for NOx emissions, the change will likely be ignored and a standard amount of injected urea will cause an increase in NOx emissions. In a time delay system that recognizes the engine out NOx spike, the system delays injecting ammonia-creating urea. Because the temperature on the SCR catalyst is relatively lower, the amount of NOx-reducing ammonia released from the catalyst is reduced, which results in a NOx emissions increase. At the end of the NOx spike event, the exhaust temperature increases (from restoration of the desired EGR fraction) while the NOx emissions decreases. The SCR catalyst ejects ammonia from the reduced storage capacity while the urea injector continues to add ammonia to the system without NOx available for reduction. Therefore, the system can slip significant amounts of ammonia on the down cycle.
Other currently available systems determine whether the SCR catalyst is at an ammonia storing (adsorption) or ammonia ejecting (desorption) temperature. When the SCR catalyst is storing ammonia, the system injects urea until the catalyst is full. When the SCR catalyst is ejecting ammonia, the system halts injection and allows stored ammonia to release and reduce NOx. Presently available systems tracking the SCR catalyst temperature suffer from a few drawbacks. For example, the amount of ammonia stored on the SCR catalyst varies with temperature. However, presently available systems assume a storage amount below a specified temperature, and zero storage above the specified temperature. Therefore, the controls may toggle significantly around the specified temperature, significantly overestimate ammonia storage capacity just below the specified temperature, and significantly underestimate ammonia storage capacity just above the specified temperature.
Several conditions, including the operation of a particulate matter filter, may negatively affect the operation of the SCR catalyst of an SCR system. For example, operation of a particulate matter filter may lead to fluctuations in the NO to NO2 ratio entering the SCR catalyst, sulphur fouling of the SCR catalyst, and hydrocarbon accumulation on the SCR catalyst, which can cause a drop in the NOx conversion efficiency and ammonia storage capacity of the SCR catalyst. These conditions are typically reversible by running the engine at a proper operating condition. The operation of a particulate matter filter (e.g., particulate matter filter regeneration events) may also cause thermal degradation of the SCR catalyst. Thermal degradation of the SCR catalyst can lead to a drop in both NOx conversion efficiency and ammonia storage capacity that is not reversible.
Some conventional engine system controllers do not account for the degradation of the SCR catalyst. Therefore, if the SCR catalyst is degraded, the NOx conversion rate commanded by the controller may be too conservative or too aggressive for a given application. Further, for conventional engine system controllers that account for the degradation of the SCR catalyst, such controllers may not account for the effect of degradation on both NOx conversion rates and ammonia storage capacity.
Other conventional SCR systems may utilize a “normalized stoichiometric ratio” (NSR) to determine baseline urea injection, but in so doing, they do not account for variances in the NOx composition and NH3 to isocyanic acid ratio of the urea when determining the NSR. Further, such systems do not account for the incomplete vaporization and hydrolysis of urea that occurs in many systems and may therefore not inject sufficient urea to reduce NOx and/or provide the desired ammonia for storage.
Also, many known SCR systems do not utilize an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst to convert at least some ammonia slipping from the SCR catalyst to N2 and other less harmful compounds. For those conventional SCR systems that do employ an AMOX catalyst, the operating conditions and conversion capability of the AMOX catalyst are not factored into the operations of the SCR systems.
The performance of aftertreatment systems are dependent upon the physical and chemical properties of the exhaust gas delivered from an internal combustion engine. The physical and chemical properties of exhaust gas are at least partially based on the engine control system's control of combustion, air-handling, and fuel. In typical systems, the engine control system, and thus the properties of exhaust delivered by an engine, does not consider the performance of the aftertreatment system in controlling the properties of the exhaust delivered by the engine. Rather, exhaust properties are controlled by the engine's control system based upon considerations exclusive of aftertreatment system performance. Accordingly, the components of conventional aftertreatment systems are configured to react to the physical and chemical properties of the exhaust gas according to desired exhaust emission targets. Often, the reactionary nature of conventional aftertreatment systems reduces the efficiency and performance of the aftertreatment system because of the transitory nature of combustion engines and inherent delays associated with broad post-combustion adjustment of the properties of the aftertreatment systems.
For example, SCR systems generate ammonia to reduce the NOx. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. Further, conventional SCR systems that utilize injected urea to produce ammonia must account for potential delays in the vaporization and hydrolysis of urea to ammonia. Additionally, SCR systems that utilize urea dosing to generate ammonia depend upon the real-time delivery of urea to the SCR catalyst as engine NOx emissions emerge. Urea dosers have relatively slow physical dynamics compared to other chemical injectors such as hydrocarbon injectors. Therefore, post-combustion adjustments in urea dosing can be delayed due to the urea doser dynamics, e.g., the degradation of the urea doser over time, of conventional SCR controls systems.
The inherent reactionary delays of conventional exhaust aftertreatment control systems are accentuated by transient and unpredicted exhaust properties associated with exhaust aftertreatment systems that do not control the exhaust properties delivered by the engine.